What is MX Record?

An MX (Mail Exchange) record is a type of resource record in the Domain Name System (DNS) that specifies which mail servers are responsible for accepting email messages on behalf of a domain. It is defined in RFC 1035 and updated by RFC 7505. Each MX record contains a preference number (also called priority) and the hostname of a mail server. Lower preference numbers indicate higher priority; the sending mail server will try the lowest-numbered server first, then fall back to higher-numbered servers if the first is unreachable. If multiple servers share the same preference value, the sender can choose among them in any order, typically by random selection for load balancing.

The record type was introduced to decouple the email receiving path from the domain's host addresses. Without MX records, email is often directed to the A or AAAA record of the domain apex, but MX records allow operators to specify mail servers on entirely different hosts or domains. The destination hostname in an MX record must resolve to one or more A or AAAA records; CNAME aliases are not permitted by the standard (RFC 2181). The wildcard MX record (e.g., *.example.com) is not recommended for mail routing because it can cause loops and unpredictable behavior.

In the wider email stack, MX records are queried during SMTP message transfer. When a Mail Transfer Agent (MTA) needs to deliver a message to user@example.com, it first asks DNS for the MX records of example.com. If no MX records exist, the MTA falls back to the A or AAAA record of the domain apex. After obtaining the server list, the MTA connects to port 25 (or 465/587 for submission) of the chosen server to deliver the message. Administrators commonly use multiple MX records with different priorities to provide redundancy and failover, a practice called secondary MX or backup MX.